System crashed but no autosave file found
It seems /tmp is cleared on startup in ubuntu by default, so
unfortunately if you had to reboot your machine everything in /tmp was erased.
The other files you saw in /tmp were probably created on startup.

Blender has some other backup systems (see this post), but if you did not make any manual saves I'm afraid there is nothing to be done.

Autosaving to the hard drive:

You could change the directory blender uses to store temporary backups to a location on your hard drive by changing the Temp location in CtrlAltU> User preferences > File:



Note that this setting controls the location of everything blender puts in /tmp by default, not just the autosave files.